It is known that when sucrose is contacted with an enzyme having fructose transferring activity (hereinafter referred to generally as fructose transferring enzyme), there is obtained a sweetener containing products of fructose transfer reaction, such as glucose as a byproduct, fructooligosaccharides which are mostly the trisaccharide (GF.sub.2), tetrasaccharide (GF.sub.3), pentasaccharide (GF.sub.4) and hexasaccharide (GF.sub.5) corresponding to sucrose coupled with 1 to 4 moles of fructose, respectively, and as a minor component, unreacted sucrose. It is further known that these fructooligosaccharides are not-substrates for the dextran sucrase produced by Streptococcus mutans, the cariogenic bacterium, and that a low-cariogenic sweetener can be produced by permitting a fructose enzyme to act on sucrose (JP-A-56-154967; the term "JP-A" used herein means "an unexamined published Japanese patent application"). It is also known that fructooligosaccharides are low-calorie sweeteners which are not digested in the living body (JP-A-58-40065) and are selectively utilized by bifidobacteria forming main part of the inetestinal bacterial flora, they can be accordingly used as selective growth factors for intestinal bifidobacteria (JP-B-59-53834; the term "JP-B" used herein means "an examined published Japanese patent application").
As the fructose transferring enzyme which can be used in the production of fructooligosaccharides, there can be mentioned the enzymes derived from strains of the genus Aspergillus such as A. niger etc., those of the genus Penicillium such as P. nigricans etc., those of the genus Fusarium such as F. lini IAM 5008 etc., those of the genus Gloeosporium such as G. kaki IAM 5011 etc., and those of the genus Aureobasidium such as A. pullulans var. melanigenum A-8 ATCC 20612 and so on. Aside from the above-mentioned microorganisms, the enzymes derived from yeasts and other microorganisms, for example strains of the genus Saccharomyces such as S. cerevisiae etc., those of the genus Rhodotorula such as R. glutinis etc., those of the genus Pichia such as P. miso etc., those of the genus Hansenula such as H. miso etc., and those of the genus Candida such as C. tropicalis etc., as well as the enzymes derived from certain plants such as asparagus, Jerusalem artichoke and so on (JP-A-56-154967 and JP-B-59-53834) can be utilized.
As an industrial process for production of fructooligosaccharides using a native fructose transferring enzyme, there is known a batch process in which a culture broth of such microorganism or the cells, disrupted cells, extract or enzyme harvested or purified therefrom is stirred together with sucrose at suitable sucrose concentration, pH and temperature (JP-A-56-154967 and 61-268190).
From the standpoint of production cost, a continuous process using an immobilized enzyme or an immobilized microorganism is preferred to a batch process with native enzyme and for this purpose, it has been proposed to immobilize microbial cells with an alginate gel by entrapping immobilization (JP-A-58-162292) or with .beta.-1,3-1,6-glucan and aluminum sulfate (JP-A-60-41497).
In order to produce fructooligosaccharides using an immobilized enzyme or an immobilized microorganism continuously on an industrial scale, it is necessary to pack the immobilized enzyme or the like into a column and pass a highly concentrated sucrose solution through the column. However, in order that this process may be successfully carried out, the immobilized enzyme or the like must have a fairly high mechanical strength. Generally speaking, an immobilized enzyme or the like obtained by gel entrapment is comparatively low in mechanical strength so that in industrial production using a highly concentrated sucrose solution as the substrate, the gel tends to be compacted to cause a progressive decrease in flow rate.
To obviate this disadvantage, entrapping immobilized microbial cells (JP-A-62-40289) and entrapping immobilized enzymes (JP-A-62-278983), having improved mechanical strength characteristics have been developed.
However, when such an immobilized preparation is packed into a column and a substrate solution is passed under industrial conditions, for example at a temperature of 50.degree. C. and a sucrose concentration of 50%, there are encountered difficulties such that the half-life of the enzyme activity is as short as about 20 days, the output and composition of the desired product vary considerably, and the withdrawal and repacking of the immobilized cells or enzyme must be carried out frequently.